An atomic force microscope (AFM) is a comparatively high-resolution type of scanning probe microscope. With demonstrated resolution of fractions of a nanometer, AFMs promise resolution more than 1000 times greater than the optical diffraction limit.
Many known AFMs include a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into contact with a sample surface, forces between the tip and the sample lead to a deflection of the cantilever. One or more of a variety of forces are measured via the deflection of the cantilevered probe tip. These include mechanical forces and electrostatic and magnetostatic forces, to name only a few.
Typically, the deflection of the cantilevered probe tip is measured using a laser spot reflected from the top of the cantilever and onto an optical detector. Other methods that are used include optical interferometry and piezoresistive AFM cantilever sensing.
One component of AFM instruments is the actuator that maintains the angular deflection of the tip that scans the surface of the sample in contact-mode. Most AFM instruments use three orthonormal axes to image the sample. The first two axes (e.g., X and Y axes) are driven to raster-scan the surface area of the sample with respect to the tip with typical ranges of 100 μm in each direction. The third axis (e.g., Z axis) drives the tip orthogonally to the plane defined by the X and Y axes for tracking the topography of the surface.
Generally, the actuator for Z axis motion of the tip to maintain a near-constant deflection in contact-mode requires a comparatively smaller range of motion (e.g., approximately 1 μm (or less) to approximately 10 μm). However, as the requirement of scan speeds of AFMs increases, the actuator for Z axis motion must respond comparatively quickly to variations in the surface topography. In a contact-mode AFM, for example, a feedback loop is provided to maintain the tip of a cantilever in contact with a surface. The tip-sample interaction is regulated by the Z feedback loop, and the bandwidth of the Z feedback loop dictates how fast scanning can occur with the Z feedback loop remaining stable.
In addition, AFMs have a number of tunable feedback loops, including the X, Y and Z feedback loops (although X and Y feedback loops are typically tuned at the factory, and not altered by the end user). Tuning the feedback loop is typically time consuming and difficult. For example, a user may manually tune the feedback loop by starting a scan with low gain, increasing some parameter until oscillation appears in the image, and then reducing the parameter until the oscillation subsides. However, the user must exercise great care because manual tuning of the feedback loop risks blunting the tip, e.g., as low gain enables the tip to crash into steps and excess gain induces positive feedback oscillations that cause the tip to repeatedly smash into the surface of the sample.
Further, the manual tuning must be repeated if anything in the AFM feedback loop is altered. For example, if the laser or the detector is realigned, then the total gain of the optical detection system will change, requiring additional tuning. Further, gradual shifts in laser power, temperature, piezoelectric constants, and the like may eventually detune or even destabilize the feedback loop. In practice, the user continually turns down the gains such that no alteration of the AFM can induce oscillations, and then takes images using very slow scan rates. In order to avoid the difficulties of manual tuning, techniques have been developed to automate tuning of the feedback loop. However, these techniques are generally complex, relatively slow, and may attempt one-time large increases in gain, which is risky in light of nonlinearities or measurement inaccuracies.